The present invention relates to a power line communication (PLC) system and particularly, to a technique of improving communication efficiency by preventing a damping of control signals superimposed on a power-supply line.
In general, built-in vehicle mountable instruments, such as various sensors and loads, are controlled by electronic control units in the vicinity of the instruments, respectively. Note that each of these electronic control units will be referred to as “slave ECU” below. In connection, the electronic control units will be referred to as “slave ECUS” below, as plural.
For instance, in case of a motor for powered window, a door ECU in the vicinity of a vehicle door is adapted so as to supply the motor with electric power and control rotation of the motor.
These slave ECUS are all connected to a main electronic control unit through wire harnesses. The main electronic control unit supplies these slave ECUS with electric power through wire harnesses (power-supply lines). Note that the main electronic control unit will be referred to as “master ECU” below.
In order to reduce the number of wire harnesses connecting these ECUS to each other, there is recently proposed a power-superimposed multiplex communication method, alias name, PLC (Power Line Communication) transmission method where control signals are superimposed on wire harnesses for power supply to communicate the signals between a master ECU and each slave ECU, being in the course of practical application.
According to the PLC transmission method, high-frequency signals are processed with ASK (Amplitude Shift Keying) modulation to produce signals in the form of “0” and “1” and successively, the resultant signals are superimposed on a power-supply line, transmitting both electric power and control signals.
Then, in the slave ECU, a PLC circuit samples the control signals superimposed on the power-supply line and demodulates the control signals based on sampling values, performing a control based on the so-demodulated control signals.
While, the master ECU is generally connected to a plurality of slave ECUS or loads through wire harnesses and is constructed so as to allow electrical power to be supplied to these elements via the wire harnesses. Here, it is noted that the control signals aren't always superimposed on all of the power-supply lines connected to the master ECU. That is, there exist some slave ECUS or loads to which only the electrical power is supplied through the power-supply lines.
In this way, the power-supply lines connected to the master ECU comprises ones having the control signals superimposed thereon and the others having no control signal superimposed thereon. Consequently, there is the possibility that the control signal after ASK modulation flows from one power-supply line having the control signal thereon into another power-supply line having no superimposed control signal, so that the control signal on the former power-supply line is attenuated to deteriorate communication efficiency.